1. Field of the Invention
This invention relates to multiple virtual displays on a single display screen, and more particularly to multiple virtual displays generated by a reentrant program on identifiable segments of a display screen.
2. Description of Prior Art
Office machines for processing text data to produce hard copies utilize display screens for presenting to an operator visual text as it would appear on hard copy. Typically, such machines incorporate a conventional typewriter keyboard with additional control keys for operator interface with the system controls. The operator keystrokes are displayed on a screen quite similar to a home television receiver screen. This screen either displays a full page of text, a partial page, or in some machines only a single line of text. The display not only provides a visual presentation to the operator of the test being prepared but also provides prompting instructions for the operator of how to interface with the machine.
Responding to the keyboard strokes of an operator are system controls including programmed computers which prepare the text display. Most present day text processing systems include electronics hardware programmed to evaluate the operator keystrokes and to control the display. This electronics hardware includes memory for storing character data and instruction data for future use in editing and the preparation of hard copy. Ease of operation of the machine for the operator is in large part determined by the electronics and the associated computers and programs.
Text processing systems utilizing a display, usually a cathode ray tube, have generally been restricted to a single image on the display screen. Thus, different messages or text presentations to an operator requires that the entire display screen be dedicated to one message or text presentation. This not only affected the efficiency of the operator and slowed down the entire text processing system, it required an operator to remember multiple messages for system operation.
Text processing systems have also been limited by processor operation parameters which defined how different programs were run to generate a display. An interrupt for a particular display is generated but the interrupt may not be serviced because of a prior commitment of the processor to service a presently running program. While the interrupts may be serviced on a priority basis, a higher priority interrupt could not gain access to the processor for operating the program until the processor was free from a presently running program. This limitation was overcome in some text processing systems by utilization of dedicated processors and memory to run in response to an identifiable interrupt. Such a solution, however, requires a considerable amount of hardware thus increasing the system cost and reducing system reliability.